Chemistry, Third edition

26 2 · ELEMENTS, COMPOUNDS AND REACTIONS

Example 2.5

Sodium reacts with water to form sodium hydroxide and hydrogen

gas. Write a balanced equation for the reaction.

Answer

1.sodiumwatersodium hydroxide hydrogen gas

2.NaH 2 ONaOHH 2

3.2Na2H 2 O2NaOHH 2

The equation in 3. is the balanced equation.

Comment

Sometimes a bit of juggling is required, but when you are balancing equations never

add subscripts to the formulae because you will change the formulae. Formulae are

fixed – only numbers in front of the formulaecan be changed in order to balance an

equation. Although equations are usually balanced using the simplest ratio of

numbers, an equation is notincorrect if higher numbers are used, as long as the

correct ratio is maintained. For example

2Na2H 2 O2NaOHH 2

means the same as

4Na4H 2 O4NaOH2H 2

or

NaH 2 ONaOH^1 ⁄ 2 H 2

because in each case the ratio of the species in the equation is 2:2:2:1.

Note that although it is acceptable to use^1 ⁄ 2 in an equation do not be tempted to

use smaller fractions – apart from this one exception, stick to whole numbers.

Writing equations

Write balanced equations for

the following reactions:

(i) The reaction of

potassium with oxygen

gas (dioxygen) to form

potassium oxide.

(ii) The reaction of iron with

chlorine gas (dichlorine)

to form iron(III) chloride.

(iii) The reaction of lithium

with water to form

lithium hydroxide and

hydrogen gas

(dihydrogen).

(iv) The reaction of

magnesium with

nitrogen gas

(dinitrogen) to form

magnesium nitride.

(v) The reaction of calcium

hydroxide with carbon

dioxide to form calcium

carbonate and water.

(vi) The reaction of sodium

hydroxide with sulfuric

acid to form sodium

sulfate and water.

(vii) The decomposition of

gold(III) oxide into its

elements (gold atoms

and dioxygen) when

heated.

(viii)The reaction of copper

with nitric acid to

produce copper(II)

nitrate, nitrogen dioxide

and water.

Exercise 2K

State symbols

Substances in balanced equations often have state symbolswritten in brackets after

them. The state symbols are:

(s) solid

(l) liquid

(g) gas

(aq) aqueous, i.e. dissolved in water

These are included to give a more detailed description of the reaction and so to aid

the reader’s understanding.

A historical perspective on this chapter is given by Box 2.6.